test, PostgreSQL - rextester

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--PostgreSQL 9.6
--'\\' is a delimiter

select REGEXP_MATCHES('a1_b_2_hello', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('2b-ffg-er2', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('abs', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('123a', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('_a1_b_2_hello', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('2b-ffg_er2-', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('__', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('--', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('a__', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');
select REGEXP_MATCHES('b--2', '^([A-Za-z0-9]*[A-Za-z][A-Za-z0-9]*(?:([-_])[A-Za-z0-9]+(?:\2[A-Za-z0-9]+)*)?$)');


absolute service time: 0,45 sec